The present invention relates to an image display for displaying images with image shift by wobbling.
Japanese Patent Disclosure No. 6-324320 discloses an image display, which comprises a shifting means disposed on an optical path between a display element having a discrete pixel array and an observing position for shifting the optical axis of light emitted from the display element in predetermined directions. In this display, odd and even field images are successively written in the same pixel of the display element for display, and in synchronism to the fields the shifting means shifts the optical axis of light emitted from the display element in predetermined directions, that is, shifts the position of the projected pixel on the display surface of the display element, thus spatially separating the odd and even field images from one another. In this way, equivalent pixels are displayed on a pixel-free black matrix portion of the display surface, thus improving resolution.
FIG. 8 shows the construction of this prior art image display. The illustrated image display comprises a color liquid crystal panel (hereinafter referred to as LCD) 1 as a display element), having a backlight 1a and a color liquid crystal display element 1b, and a means including a polarization converting element 2 and a double refractor 3 disposed one in front of the other on the front surface side of the LCD 1. The LCD 1 has, for instance, one half the scanning lines of the NTSC, and as shown in a fragmentary plan view in FIG. 9, has delta arrays of R, G and B pixels. In FIG. 8, a reduced number of, i.e., several, scan lines are shown for the sake of the clarity of the drawing.
As the polarization converting element 2, a twist nematic liquid crystal shutter (hereinafter referred to as TN shutter) is usually used, which is relative inexpensive and is manufactured on the basis of an established technique. As shown in FIGS. 10(a) and 10(b), the TN shutter 2 includes a pair of polarizing members 6 having transparent electrodes 5 and a TN liquid crystal layer 7 sandwiched between the transparent electrodes 5. An AC power source 9 is connected between the pair transparent electrodes 5 via a switch 8. As shown in FIG. 10(a), with an AC voltage applied across the TN liquid crystal layer 7 by turning on the switch 8, the polarization of light incident on the polarization converting element 2 is transmitted without being rotated. As shown in FIG. 10(b), with no AC voltage applied across the TN liquid crystal layer 7 by switching off the switch 8, the polarization of the incident light is transmitted while it is rotated by 90 degrees.
The double reflector 3 is formed from an anisotropic crystal, such as rock crystal (xcex1-SiO2), lithium niobate (LiNbO3), rutile (TiO2), calcite (CaCo3), Chile nitre (NaNo3) and YVO4. As shown in FIG. 11, it transmits incident light of a first polarization as normal light, and transmits incident light of a second polarization at right angles to the first polarization as abnormal (shifted) light. Denoting the thickness of the double refractor 3 in z-axis direction perpendicular to xy coordinates of the display surface of the color LCD 1, i.e., the direction of incidence of light beam by d and the angle of separation between the normal light and abnormal light by xcex8, the normal and abnormal light beams emitted from the double reflector 3 are spatially separated by dxc3x97tanxcex8.
Thus, with the crystallization axis 3a of the double refractor 3 set in a suitable direction, as shown in FIG. 12, by turning off the TN shutter 2 the polarized light is rotated in the TN shutter 2 by 90 degrees and transmitted therethrough as a second polarized light, and is then transmitted through the double refractor 3 as, for instance, abnormal light. In this way, as shown in FIG. 13, the pixels of the display surface of the color LCD 1 can be observed in black matrix positions which are obliquely upwardly and rightward from their original (non-shifted) position by substantially one half pixel pitch from the original pixel positions. As shown in FIG. 14, by turning on the TN shutter 2 the polarized light from the color LCD 1 is transmitted through the TN shutter 2 without being rotated but as the input first polarized light itself, and is transmitted through the double refractor 3 as normal light. In this case, the pixels of the display surface of the color LCD 1 can be observed in their original positions as shown in FIG. 9.
In the prior art image display as shown in FIG. 8, the properties of the TN shutter 2 and the double refractor 3 are utilized such that, while odd and even field images of the input image signal are successively displayed on the same pixel of the color LCD 1 under control of an image display control circuit 11, the voltage applied to the TN shutter 2 is on-off controlled fixedly by an TN shutter drive circuit 12 which constitutes a vibrating means. Thus, pixel shifting, i.e., changing of the pixel position observed through the double refractor 3 according to the direction of polarization of light transmitted through the TN shutter 2, is obtained to improve the resolution. More specifically, in the odd field the TN shutter 2 is held xe2x80x9coffxe2x80x9d, and, as shown in FIG. 15, the observed pixel positions are shifted obliquely upwardly and rightward by substantially one half pixel pitch from the original pixel positions (the shifted pixel positions in this case being shown as Ro, Go and Bo). In the even field the TN shutter 2 is held xe2x80x9conxe2x80x9d, and, as shown in FIG. 16, the original pixel positions are restored as the observed pixel positions (the pixel positions in this case being shown as Re, Ge and Be). It is thus possible to permit observation of images with double the pixel number of the color LCD 1.
For the odd and even field images displayed on the color LCD 1, the image signal is sampled at timings which are different from each other by a time corresponding to the extent of image shift. More specifically, when displaying the odd field images, the timing of sampling of the image signal is delayed to be behind the timing of when displaying the even field images by a time corresponding to substantially one half pixel pitch. Also, since the color LCD 1 holds the entire image on the display until it is re-written by the next field image, one of the pair electrodes of the TN shutter 2 is divided into a plurality of lines, for instance about 5 lines, while the other electrode is used as a common electrode. The voltage application is thus controlled by selecting the divided electrodes according to the timing of the line scanning of the color LCD 1.
However, various experiments conducted by the inventor with the prior art image display adopting the pixel shifting technique described above, reveal that when the TN shutter 2 is on-off controlled fixedly at the same timings as those of switching of the odd and even field images displayed on the color LCD 1, i.e., at an interval of {fraction (1/60)} second, sufficient resolution improvement can not be obtained due to influence of the response characteristic in the rotation of the polarized light from the TN shutter 2.
FIGS. 17(a) and 17(b) are views for describing the response characteristics in the rotation of the polarized light in the TN shutter 2. Specifically, FIG. 17(a) shows the first polarized light transmittance, and FIG. 17(b) shows the drive voltage. It is assumed that a high frequency voltage is applied as the drive voltage. The TN shutter 2 has a rise response time xcfx84ON when the drive voltage is turned on and a fall response time xcfx84OFF when the drive voltage is turned off. Denoting the maximum and minimum first polarized light transmittances of the TN shutter 2 by Tm and To, respectively, the rise response time xcfx84ON is represented by the sum of a rise delay time tdON from the instant when the drive voltage is turned on until the instant when 10%, i. e., (To+0.1 (Tmxe2x88x92To)) is reached by the first polarized light transmittance after the commencement of behavior of the liquid crystal and a rise time tr from the instant of actual rising of the TN liquid crystal upon reaching of the 10% transmittance till the instant of reaching of 90% transmittance, i.e., (To+0.9(Tmxe2x88x92To)). The fall response time xcfx84OFF, on the other hand, is represented by the sum of a fall delay time tdOFF from the instant of commencement of the behavior of the liquid crystal when the drive voltage is turned off till the instant of transmittance fall down to 90% and a fall time td from the instant of actual falling of the TN liquid crystal upon the transmittance fall down to 90% till the instant of the transmittance fall down to 10% again.
In the above response characteristics, the values of tdON, tr, tdOFF and td are, for instance, tdON=0.5 ms, tr=1 ms, tdOFF=5 ms and td=5 ms. The rise time tr depends on the applied drive voltage, while the fall time td depends on material characteristics peculiar to the liquid crystal. With the rise and fall response times xcfx84ON and xcfx84OFF being different from each other, the on-off switching of the TN shutter 2 at the same timings (the same instants) as those of the field switching, results in different transmitted light characteristics in the xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d periods of the TN shutter 2. Therefore, the residual light in the preceding field deteriorates the contrast, thus making it impossible to improve the resolution by the pixel shifting.
FIGS. 18(a) and 18(b) show the first polarized light transmittance of the TN shutter 2 and on/off timings of the drive voltage, respectively, for a single pixel of the LCD display. It will be seen that, although in the even fields only the non-shifted pixels should be transmitted to display the data thereof (i.e., image signal data Re, Ge and Be) in the original pixel positions (hereinafter also referred to as even lines), the second polarized light is also transmitted during the rise response time xcfx84ON. Therefore, the first polarized light data is also displayed in the shifted pixel positions (hereinafter also referred to as odd lines), in which intrinsic data of the second polarized light (i.e., image signal data Ro, Go and Bo) are to be displayed, thus resulting in an image as shown in FIG. 19(a) which is observed. Likewise, the first and second polarized light beams are also transmitted during the fall response time xcfx84OFF. Therefore, the intrinsic odd line data (i.e., image data Ro, Go and Bo) is also displayed in the even lines, thus resulting in an image as shown in FIG. 19(b) which is observed.
As a result, it is impossible to obtain sufficient improvement of the resolution by pixel shifting. In addition, due to the fact that the fall response time xcfx84ON is long compared to the rise response time xcfx84OFF, a problem of contrast deterioration is posed. In the case of FIGS. 18(a) and 18(b) opposite polarity drive voltages are alternately applied to the TN shutter 2 in the successive even fields. However, it is also possible to apply a high frequency drive voltage in each voltage application period.
The calculation of the contrast will now be described with reference to FIGS. 20(a) to 20(b), which are enlarged-scale views of FIGS. 18(a) and 18(b). In FIG. 20(a), the response characteristic in the rotation of the polarized light is approximated by a straight line plot for the sake of simplifying the calculation. In FIG. 20(a), denoting the areas of the even and odd fields of the response characteristic by Se and So, respectively, the contrast Cont is given as:
Cont=(Sexe2x88x92So)/(Se+So)xe2x80x83xe2x80x83(1)
Denoting the time of one field by tF, the areas Se and So are given as:
Se=tFxe2x88x92xcfx84ON+(xc2xd)trxe2x80x83xe2x80x83(2)
and
So=tdOFF+(xc2xd)td.xe2x80x83xe2x80x83(3)
The contact Cont is thus given as:
Cont=(tFxe2x88x92xcfx84ONxe2x88x92tdOFF+(xc2xd)(trxe2x88x92td))/(tFxe2x88x92xcfx84ON+tdOFF+(xc2xd)(tr+td))xe2x80x83xe2x80x83(4)
By substituting tF=16.67 ms (i.e., {fraction (1/60)} s) and also the values described before in connection with FIG. 17, i.e., tr=1 ms, tdOFF=5 ms, td=5 ms and xcfx84=1.5 ms, respectively, we have
Cont=0.353
In other words, in the image display shown in FIG. 8 the contrast is reduced by about 65% due to the response characteristic of the NT shutter 2 to the rotation of the polarized light.
The present invention was made in view of the above problems inherent in the prior art, and its object is to provide an image display, which has an adequate structure for effectively preventing the deterioration of the contrast and permitting sufficient resolution improvement to be obtained by pixel shifting.
According to an aspect of the present invention, there is provided an image display comprising a display element having a display surface with a regular array of a plurality of pixels, a pixel shifting means for shifting an optical axis of a light beam emitted from the display surface in a predetermined direction, and an image display control means for causing display of different images on the display element in synchronism to the shifting of the optical axis caused by the pixel shifting means, wherein the pixel shifting means includes a polarization conversion control means capable of controlling polarization conversion timings.
The polarization conversion control means includes a polarization conversion element and a driving means for driving the polarization conversion element according to a response characteristic thereof. The driving means is capable of setting both the timing of switching the xe2x80x9conxe2x80x9d times of drive signals for transmitting a first and a second polarized light beams (i.e., the rotated and non-rotated beams) to the polarization conversion element and the ratio between the two xe2x80x9conxe2x80x9d times. The drive signal switching timing and the time ratio are set such that the transmitted doses of the first and second polarized light beams at the time of the polarization conversion are substantially 50%. The driving means includes a plurality of delayed signal generators for generating delayed signals according to a synchronizing signal of image displayed on the display element.
According to another aspect of the present invention, there is provided an image display which includes display element means having a display surface with a plurality of pixels and polarization converting means opposed to the display element means for rotating a light beam emitted from the display surface in a predetermined direction in response to a drive signal, and shifts an optical axis of the light beam in a predetermined direction and successively displays on the same image pixel images of an odd field and a even field, wherein the drive signal is determined on the basis of a response characteristic of the polarization converting means.
The xe2x80x9conxe2x80x9d times of the drive signals for transmitting a first and a second polarized light beams from the polarization conversion means and the time ratio between the two xe2x80x9conxe2x80x9d times are settable. The switching timing and the time ratio by the drive signal are set such that the transmitted doses of the first and second polarized light beams at the time of the polarization conversion are substantially 50%.
According to other aspect of the present invention, there is provided an image display which includes display element means having a display surface with a plurality of pixels, and polarization converting means for rotating a light beam emitted from the display surface in a predetermined direction in response to a drive signal and a double refractor disposed one in front of the other on the front surface side of the display element means, and successively displays on the same image pixel images of an odd field and a even field, wherein the drive signal is determined on the basis of a response characteristic of the polarization converting means and the image pixel positions observed via the double refractor are shifted.
The polarization converting means is on-off controlled such that at the time of the switching of fields the first and second polarized light transmittance are substantially 50% when one image pixel is considered. The on-off timings and duty ratio of the drive signal are determined. The polarization conversion means is TN shutter having a short rise response time compared to the fall response time. The display element is color LCD, monochromatic LCD, plasma display, EL or photochromics. The display element has a delta array, stripes arrays or mosaic arrays, and the image position shift by the wobbling is obtained through interpolation in conformity to the image pixel array. The image display further comprises temperature detector for detecting the temperature in the neighborhood of the polarization converting means and control means for controlling the on-off timings of the drive signal and the duty ratio on the basis of the detected temperature.
Other objects and features will be clarified from the following description with reference to attached drawings.